One type of antenna commonly used with mobile devices is a PIFA antenna. PIFA antennas typically include a ground plane, a top plate element, a feed wire feeding the resonating top plate, and a DC-shorting plate that connects the ground plane and one end of the resonating plate. An impedance element also can be included between the ground plane and the resonating plate. PIFA antennas generally are designed to work around one band of frequencies and typically display “nulls” in frequencies outside of that frequency band.
In some IEEE wireless communication standards, MIMO (multiple-input multiple-output) devices can use more than one transmitting and receiving antenna, the transmitting and receiving antennas being physically separated, with the effect that multiple signals can be transmitted and received concurrently using the same communication channel. For example and without limitation, a 1st MIMO device having antennae 1a and 1b can communicate with a 2nd MIMO device having antennae 2a and 2b, using a substantially single communication channel, by communicating between antennae 1a and 2a and between antennae 1b and 2b. Alternatively, the 1st and 2nd MIMO devices might communicate between antennae 1a and 2b and between antennae 1b and 2a. Communication channels are described herein primarily with respect to distinct carrier frequencies; however, in the context of the invention, there is no need for any particular limitation. For example and without limitation, communication channels might include CDMA or TDMA access to a common communication medium.
One known problem in MIMO antenna design is to substantially reduce correlation between and among received signals at the receiving end of a pair of communicating devices' antennae. While this is relatively easy to achieve in a scattering-rich environment, an environment that is not so conducive to MIMO operation is subject to drawbacks when the antennae themselves do not exhibit operational diversity. Moreover, in IEEE 802.11 protocols which use MIMO to advantage, it is relatively difficult to achieve the advantages of MIMO operation concurrently with respect to more than one communication channel, as antennae that are relatively effective for MIMO operation for a 1st communication channel, such as for example a 1st frequency, can be subject to substantial inefficiency for MIMO operation for a 2nd communication channel, such as for example a 2nd frequency. For example, standard PIFA antennas tend not to be able to operate in both 2.4 GHz and 5.0 GHz channels. This can pose a significant drawback in IEEE 802.11 protocols in which MIMO operation in combined with operation using more than one carrier frequency.